Video information and graphical information are being combined in an increasing number of applications. Examples include closed captioning of television transmissions, on-screen menus, and television displays that can switch to display a browser for the world-wide web. Typically in these applications, the video information is generated separately from the graphical information and the two must be combined before being outputted to the display device.
In some applications, such as set-top boxes intended to work with television sets in a home environment, the video information is of paramount importance. This is understandable in applications such as digital video discs and digital television where a live video feed is being displayed, and may be displayed in conjunction with graphics information. In such cases, in order to avoid glitches in the video display, the system must ensure that enough bandwidth is provided to fetch and display the video information.
One of the problems associated with combining video and graphics information in a video-intensive application is that the graphics information puts additional loading on the bandwidth and other capabilities of the system. For example, in a system where both the video information and the graphics information is stored in a single memory structure, the fetches of graphical information may interfere with the fetching of video information. In these systems, it is important to minimize the impact that the graphics information retrieval has upon the video information retrieval.
Another problem that can arise in mixed video and graphics circuitry is the amount of memory required to store the video and graphics images. A typical system will allocate an appropriate amount of memory for both the video information and the graphical information based on the size and resolution of the display screen. For example, if a screen is 640.times.480 pixels, memory will be allocated to display video information at each of those pixel locations as well as graphical information at each of those pixel locations. This is because the actual pixel display is generated based on the combination of the graphics and video information. In many cases this allocation scheme is wasteful as graphics information may only be relevant or present within a small portion of the display.
Therefore a need exists for a method and apparatus that allows video and graphics information to be displayed simultaneously on a screen while improving the efficiency of memory usage with respect to the storage and fetching of graphics information.